The cotton rat was used as an experimental animal to test the protective efficacy of passively acquired serum neutralizing antibodies to RS virus (RSV). Infant cotton rats inoculated with varying amounts of homologous convalescent RSV antiserum were subsequently challenged intranasally with RSV. Cotton rats with a circulating titer of neutralizing antibody of 1:350 or greater at the time of challenge were completely resistant to pulmonary infection; cotton rats with a titer of 1:80 to 1:350 became infected but the level of virus replication in the lung was reduced in direct relation to the titer of antibody; while animals with an antibody titer less than 1:80 sustained an undiminished RSV infection in the lungs. The nose was not protected against RSV even by the highest level of passive serum antibodies. These observations suggest that maternally-derived antibodies can exert a protective effect against RSV infection in the lungs, however, a very high concentration of antibodies is required for significant protection in the lower respiratory tract. The concentration of passive serum antibodies in the circulation of young human infants decreases approximately 50% every month. This may serve to explain the relative sparing of very young infants from serious RSV disease. Within a few months after birth the concentration of passively aquired RSV antibodies drops below the critical threshold, 1:80-1:350, and the lungs lose their resistance. Suspensions of purified human IgG (Sandoglobulin) were shown to contain a very high level of antibody to RSV. Sandoglobulin administered intraperitoneally to cotton rats prevented pulmonary infection with RSV. Possibly more significant was the therapeutic effect of human RSV antibodies administered after infection had become well established. Administration of lesser amounts of Sandoglobulin to RSV-infected owl monkeys also decreased the level of virus in the lungs over the next 2 days. These observations suggest that serious RSV disease in infants may respond to immunotherapy.